In a known test circuit for testing the correct functioning of a passenger restraint system, described in the U.S. Pat. No. 3,911,391, the output stage of the system which normally actuates the restraint system is blocked during the test program. Accordingly, it is impossible to detect any possible defect in the output stage. In some passenger restraint systems, for example the so-called air bag restraints, in which a collision of the vehicle with an obstacle results in the automatic inflation of an air bag to protect the occupants, an intentional release can be as dangerous as a failure to deploy. Accordingly, the entire system including the output stage should be checked at regular intervals.